The entire disclosure of any publication or patent document mentioned herein is incorporated by reference, including U.S. patent application Ser. No. 14/707,841 filed May 8, 2015 and PCT Patent Application Serial No. PCT/US14/64272 filed Nov. 6, 2014.
Optical fiber transmission systems are employed in data centers to establish communication between devices such as routers, servers, switches and storage devices. The optical fiber transmission system typically utilizes a trunk cable (e.g., tens to hundreds of meters long) that carries many optical fibers (e.g., twelve, twenty-four, forty-eight, etc.). Each end of the trunk cable optically connects to a breakout assembly to transition from MPO-style multifiber trunk connectors to other types of connectors, which are then interfaced with patch cords or plugged directly into equipment ports, thereby establishing an optical path between the devices. The breakout assembly is frequently housed in a break-out module.
The optical fibers used in data center applications are typically multimode optical fibers (MMFs) because the light sources in the transceivers in the optical devices are typically multimode light sources (transmitters).
Light emitted from the multimode transmitter has a distribution across the core area. For VCSEL based application, IEEE has defined the launch conditions that need to be in compliance with. The distribution is in general weighted more in certain region. The requirement for VCSEL emission for Ethernet application is that at 4.5 micron radius, the cumulated or integrated optical power from the center of the fiber core should be less than 30% of the total optical power. At 19 micron radius position, the integrated optical power should be over 86%. The MMFs are designed with the launch condition taken into consideration and the launch condition determines the bandwidth number for a given MMF. Silicon-photonics (SiPh) light sources can also have a non-uniform light distribution wherein the intensity of the emitted light is greater towards the outer edge of the light source than at the center. Depending on design and implementation of coupling optics between the transmitter output and the MMF interface, more light can be launched in the outer portion of the core of the MMF where the higher-order modes (HOMs) travel than in the desired central or inner portion.
This enhancement or “amplification” of the HOMs is undesirable because can lead to transmission problems, including a reduction in the system bandwidth. For example, for wavelength division multiplexing (WDM) applications (and in particular, coarse WDM) that operate at several different wavelengths in the range from 850 nm to 950 nm, 980 nm to 1060 nm, or 1270 nm to 1330 nm, the fiber bandwidth is more limited for light source with light spread across the whole MMF core area, thereby reducing the MMF product yield and system reach capability. In addition, MMF bandwidth is typically measured at lengths of greater than 8.8 km and more frequently at 17.6 km. The HOMs that travel in the MMF are significantly attenuated only at these long distances. Thus, the differential mode delay (DMD) measurement and the calculated modal bandwidth can differ substantially as compared to the shorter MMF lengths used in data center applications.
It would thus be advantageous to have ways of improving the performance of a multimode optical fiber transmission system without incurring the time, labor and expense of having to replace or physically alter the industry-standard MMFs.